The present invention relates to a broad beam ultrasonic transducer.
U.S. Pat. No. 4,677,377 discloses a piezoelectric transducer which is intended to be used as an emitting transducer or as a receiving transducer for ultrasonic waves propagating in air. The use of the transducer disclosed by this publication has solved substantial problems which are associated with the extreme difference between the acoustic wave impedance of the sound-transmitting medium of air and the acoustic wave impedance of a solid body emitting or receiving the ultrasonic waves. This acoustic wave impedance is also referred to as the acoustic characteristic impedance.
The ultrasonic transducer of the aforementioned publication has an acoustic characteristic impedance which in relative terms is substantially closer to the value thereof of air. This is achieved by a sandwich construction which consists of individual mutually spaced piezoelectric laminae disposed in planes parallel to one another, the intermediate spaces, corresponding to the spacings, between these laminae being filled with an inherently stable material which has a low acoustic characteristic impedance value. The material occupying the intermediate spaces forms at least one closed surface of this electroacoustic transducer enclosing the piezoelectric laminae, namely a surface for the emission and/or for the reception of acoustic radiation. In this case, for example, this material occupying the intermediate spaces may extend beyond at least a respective one of the edge surfaces of the individual laminae, so that these edge surfaces of the laminae are covered in relation to the external environment by this material occupying the intermediate spaces.
Such a known transducer may be designed so that this surface of the same which is provided for emission and/or reception has relatively large dimensions as compared with the wavelength, in air, of the emitted or received acoustic radiation. If the individual piezoelectric laminae are excited to execute co-phase oscillation, then, originating from this surface of the transducer, an acoustic wave with a substantially plane phase front is emitted.
The material employed for the laminae is piezoelectric ceramic, e.g. lead zirconate titanate, lead titanate, barium titanate and the like, it being possible for these materials to include dopings and/or substitutions, of, inter alia, manganese, niobium, neodymium etc. to improve their respective properties. The material intended to occupy the intermediate spaces between the laminae is, in this known transducer, for example a thermoplastic material. By way of example, the entire body consisting of this material and the piezoelectric laminae is adhesively bonded together while hot. However, the intermediate spaces in this body may also be filled with a sealing compound consisting of silicon rubber.
With regard to further details with respect to the structural configuration and the production of such a known transducer, reference is made to the aforementioned publication.